H.264 encoding settings for High Quality HD (1080p) and High Quality UHD (4K)
Unfortunately H264 is never going to give you brilliant quality video. It was designed to be efficient on storage and network bandwidth. Take a look at mjpeg or similar for higher quality video.

GOP stands for Group of pictures. And basically means how many frames between I frames. Generally you would have 1 I frame to 24 p frames so the GOP would be 25. The more I frames you have the higher quality picture but the higher storage and network cost too. 

I frame is an interframe and is a complete picture. P frames are predicted frames and made up of the predicted changes between the last I frame and the next frame.

Limiting the max bitrate will cause Blender to over compress the image and movement or dark areas will look worse.  I can't see any benefit in setting the minimum bitrate